antagonist

an·tag·o·nist

noun \-nəst\

Definition of ANTAGONIST

: an agent that acts in physiological opposition <contact between a tooth and its antagonist in the opposing jaw>: as a : a muscle that contracts with and limits the action of an agonist with which it is paired—called also antagonistic muscle; compare agonist 1, synergist 2 b : a chemical that acts within the body to reduce the physiological activity of another chemical substance (as an opiate); especially : one that opposes the action on the nervous system of a drug or a substance occurring naturally in the body by combining with and blocking its nervous receptor—compare agonist 2